binding: navigation: move get_storage_from_value call within write lock

json_object returned from get_storage_from_value() needs to be protected
within the write lock. This is to prevent reentrant calls to broadcast() from
accessing it, and running json_object_put()

Bug-AGL: SPEC-2880
Change-Id: I9fdf614728368c75e925e4c5a0f3ca19a7e517e1
Signed-off-by: Matt Ranostay <matt.ranostay@konsulko.com>